The present invention relates to a battery and a process for preparing the same. More particularly, the present invention relates to a battery in which safety is ensured by controlling temperature rise caused by short-circuit or the like, and a process for preparing the same.
Recently, with development in electronic appliances, high leveling of capacity and output density of a battery used as a power source is being advanced. As a battery, which can satisfy these requirements, attention is paid to a lithium ion secondary battery. The lithium ion secondary battery has an advantageous effect that energy density is high, while a sufficient counterplan for safety is required because a non-aqueous electrolytic solution is used.
As a counterplan for safety it has been conventionally suggested to incorporate a safety valve which releases increased internal pressure, or a PTC device of which resistance increases in accordance with the heat generated from external short circuit to break an electric current.
For example, as disclosed in Japanese Unexamined Patent Publication No. 328278/1992, there is known a method for attaching a safety valve and a PTC device to the positive electrode cap of a cylindrical battery. However, when the safety valve is operated, water in air may invade into the battery to react with lithium in the negative electrode and there is a fear of an exothermic reaction.
On the other hand, the PTC device successively breaks external short-circuit without causing any troubles. As a safety component running firstly at the emergency of the battery, the PTC device can be designed to run when the battery reaches at least 90xc2x0 C. due to external short circuit.
Since the conventional lithium secondary battery has the structure mentioned above, there exist the following problems.
At occurrence of short-circuit and temperature rise inside the lithium secondary battery, increase of the short-circuit current can not be controlled in a conventional lithium secondary battery.
When the short-circuit inside the lithium secondary battery increases a temperature, a polyethylene or polypropylene separator interposed between the positive electrode and the negative electrode is expected to have a function that the separator softens or melts to close holes thereon and release or seal a non-aqueous electrolyte contained in the separator to decrease its ion conductivity, and thereby reducing the short-circuit current.
But in case of using a solid electrolyte or a gel electrolyte instead of these separators or if a lithium battery is prepared without using separator, there is no component for decreasing short-circuit due to softening and melting of the separator, and an alternative safety component is necessary.
Besides, particularly in a lithium ion secondary battery, a negative electrode is formed by applying a slurry comprising a negative electrode active material such as graphite, a binder such as PVDF (poly(vinylidene fluoride)) and a solvent, onto a substrate such as a copper foil which forms a collector, and drying it to form a thin film thereof. A positive electrode is formed as a thin film in the same manner onto a sabstrate such as an aluminum foil which forms a current collector. The positive electrode contains a positive electrode active material such as LiCoO2, a binder and a conductive agent.
The conductive agent is used to increase an electronic conductivity at a positive electrode when the positive electrode active material has insufficient electronic conductivity. As the conductive agent, there is used carbon black (such as acetylene black) or graphite (such as artificial graphite KS-6 available from LONZA Co., Ltd.).
As mentioned above, such a battery has a problem that when a temperature of the battery increases due to internal short-circuit or the like, there is no component for decreasing short-circuit current, and large short-circuit current is generated, and thus temperature of the battery further increases due to generation of heat, leading to a further increase of short-circuit current.
The present invention has been carried out in order to solve the above problems. The object of the present invention is to provide a highly safe battery in which the increase of short-circuit current can be controlled even at temperature rise caused by generation of heat due to short-circuit by constructing the battery with an electrode in which resistance increases in accordance with temperature rise.
The first battery of the present invention comprises a solid electrolytic layer between a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode comprises an active material layer containing an active material and an electronically conductive material contacted to the active material, and wherein the electronically conductive material comprises an electrically conductive filler and a resin so that resistance increases with temperature rise. According to this, since the above electronically conductive material contains the electrically conductive filler and the resin to increase resistance thereof with temperature rise, increase of current flowing into the electrode can be controlled when temperature increases due to generation of heat with short-circuit or the like and there is obtained a highly safe battery.
The second battery of the present invention comprises a gel electrolytic layer between a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode comprises an active material layer containing an active material and an electronically conductive material contacted to the active material, and wherein the electronically conductive material comprises an electrically conductive filler and a resin so that resistance increases with temperature rise. According to this, since the above electronically conductive material contains the electrically conductive filler and the resin to increase resistance thereof with temperature rise, increase of current flowing into the electrode can be controlled when temperature increases due to generation of heat with short-circuit or the like and there is obtained a highly safe battery.
The third battery of the present invention comprises an electrolyte-containing porous layer comprising fine particles between a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode comprises an active material layer containing an active material and an electronically conductive material contacted to the active material, and wherein the electronically conductive material comprises an electrically conductive filler and a resin so that resistance increases with temperature rise. According to this, since the above electronically conductive material contains the electrically conductive filler and the resin to increase resistance thereof with temperature rise, increase of current flowing into the electrode can be controlled when temperature increases cue to generation of heat with short-circuit or the like and there is obtained a highly safe battery.
The fourth battery of the present invention is that in any of the above first to third batteries, the resin contains a crystalline resin. According to this, by containing the crystalline resin in the resin, the rate of increase in resistance with temperature rise (changing ratio of resistance) can be improved, and there is obtained a battery capable of rapidly controlling increase of current flowing into the electrode when temperature is increased.
The fifth battery of the present invention is that in any of the above first to third batteries, a melting point of the resin is in the range of 90xc2x0 C. to 160xc2x0 C. According to this, by using the resin having a melting point of 90xc2x0 C. to 160xc2x0 C., the electronically conductive material can increase changing ratio of resistance at about a pre-determined temperature of 90xc2x0 C. to 160xc2x0 C., and thus characteristics of battery and safety can be coexistent with each other.
The sixth battery of the present invention is that in any of the above first to third batteries, 0.5 to 15 parts by weight of the electronically conductive material is contained in 100 parts by weight of the active material. According to this, by using the active material layer containing 0.5 to 15 parts by weight of the electronically conductive material in 100 parts by weight of the active material, resistance of the electrode before increase of changing ratio of resistance against temperature can be lowered and discharging capacitance of the battery can be increased.
The seventh battery of the present invention is that in any of the above first to third batteries, an amount of the electrically conductive filler is 40 to 70 parts by weight in the electronically conductive material. According to this, by setting the amount of the electrically conductive filler to 40 to 70 parts by weight in the electronically conductive material, changing ratio of resistance with temperature rise can be increased and normal resistance can be lowered. At the same time, discharging capacitance of the battery can be increased.
The eighth battery of the present invention is that in any of the above first to third batteries, the electronically conductive material has a particle size of 0.05 xcexcm to 100 xcexcm. According to this, by setting the particle size of the electronically conductive material to 0.05 xcexcm to 100 xcexcm, resistance of the electrode before increase of changing ratio of resistance against temperature can be lowered and discharging capacitance of the battery can be increased.
The ninth battery of the present invention is that in any of the above first to third batteries, a carbon material or an electrically conductive non-oxide is used as the electrically conductive filler. According to this, since the carbon material or the electrically conductive non-oxide is used as the electrically conductive filler, the electric conductivity of the electrode can be improved.
The tenth battery of the present invention is that in any of the above first to third batteries, the positive electrode contains a conductive agent. According to this, since the positive electrode contains the conductive agent, resistance of the electrode can be suitably controlled even in case of using the electronically conductive material having a small electronic conductivity.
The first process for preparing the battery of the present invention comprises the steps of:
(a) forming fine particles of the electronically conductive material by pulverizing an electronically conductive material comprising an electrically conductive filler and a resin;
(b) preparing an active material paste by dispersing the above fine particles of the electronically conductive material and an active material in a dispersion medium;
(c) forming an electrode by drying the above active material paste and by pressing it at a predetermined temperature T1 and a predetermined pressure; and
(d) layering and laminating one of a solid electrolytic layer, a gel electrolytic layer and an electrolyte-containing porous layer comprising fine particles to the electrode.
According to this, since it comprises the steps (a) to (d), there can be prepared a battery which controls the increase of current flowing into the electrodes with temperature rise. Moreover, since this process includes the step (c), the adhesion between the electronically conductive material and the active material becomes high and the resistance of the prepared electrode can be controlled into a low value.
The second process for preparing the battery of the present invention is that in the first process, the resin contains a crystalline resin. According to this, by containing the crystalline resin in the resin, the rate of increase in resistance to temperature rise (namely, changing ratio of resistance) can be improved, and there is obtained a battery capable of rapidly controlling increase of current flowing into the electrode when temperature increases.
The third process for preparing the battery of the present invention is that in the first process, a predetermined temperature T1 is a melting point of the resin or a temperature near the melting point. According to this, by setting the predetermined temperature T1 to the melting point of the resin or the temperature near the melting point, the adhesion between the electronically conductive material and the active material is further improved and resistance of the prepared electrode can be further decreased.